A large class of linear voltage regulators provides a regulated voltage by way of a feedback loop comprising an operational amplifier and a pass transistor. An example of a linear voltage regulator is illustrated in FIG. 2. As is well known, a negative feedback loop regulates the voltage at node 202 to match a reference voltage VREF, where the feedback loop is formed by the output port of amplifier A connected to the gate of pass transistor Q, and the drain of transistor Q connected to the positive input port of amplifier A. The reference voltage VREF is applied at the negative input port to amplifier A. Load 204 is the circuit for which a regulated voltage is desired, and capacitor 204 is a de-coupling capacitor. Load 204 may be, for example, a circuit within a microprocessor. Particular examples include, but are not limited to, a phase locked loop, a delay locked loop, or a thermal sensor.
Let ZREG denote the small-signal impedance presented by the linear voltage regulator to voltage rail 204. It has been observed that there may be an undesirable interaction between the supply voltage Vcc at voltage rail 204 and the linear voltage regulator of FIG. 2. In particular, it has been observed that if the phase of the impedance ZREG falls below −90 degrees, there may be spontaneous oscillations at voltage rail 204. This problem is more likely to worsen as the number of linear voltage regulators connected to voltage rail 204 increases, as for example in applications in which there are more than one microprocessor core or more than one I/O (Input/Output) channel.
A linear voltage regulator of the type illustrated in FIG. 2 is generally designed so that the poles of its closed-loop transfer function are the zeros of its impedance ZREG. This results in the phase of the impedance ZREG being less than −90 degrees, unless the linear voltage regulator is designed to be over-damped. However, such an over-damped design is not necessarily trivial or desirable for some applications, as it generally requires a relatively large capacitor for compensation. Furthermore, such a relatively large capacitor results in a linear voltage regulator with a low operating bandwidth. A low operating bandwidth linear voltage regulator may need a large output de-coupling capacitor to provide adequate power supply rejection (PSR). But large output de-coupling capacitors are not necessarily desirable because of their size, and because of possible current leakage.